1. Manual tabs. Take them all out and replace by setting a first line automatic indent of .3-.5.
2. Standard headers. You added your name and the page number when you submitted to agents or publishers. Delete it all. You don't need page numbers with an eReader.
3. Double spacing. Single spacing is preferred.
4. Multiple styles. Change everything to "Normal" style except for chapter titles which should be "Heading" style.
5. Too large font size. 10 point is large enough with 11 or 12 for title and chapter headings.
6. Unconventional fonts. For best results use something that translates well into HTML like Times New Roman or Verdana.
7. Extra paragraph returns. Also, no paragraph returns between paragraphs unless you want a purposeful break and never at the end of a line.
8. Auto-generated Your Table of Contents. Instead hyperlink each heading to manually build the TOC.
9. Tables or Columns to format text. A disaster waiting to happen. Try to remove them.
10. Margin size greater than 1 inch all around. Don't do it. You won't like the results.
I hope these are helpful to first timers. Any other tips from veteran ePubbers out there?